Method for enhancing the metallurigcal quality of products treated in a furnace

ABSTRACT

The method and apparatus for enhancing the metallurgical quality of products treated in a furnace with several zones, wherein the temperature and the atmospheric conditions can be controlled. The applies to any type of product treated in a furnace, such as billets, blooms, slugs or slabs. Alternatively, this may be used by iron and steel manufacturers in the production line for sheets, plates, tubes, etc.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for enhancing the metallurgicalquality of products treated in a furnace, and especially a reheatfurnace. This invention applies to any type of product, but moreparticularly to products treated in a reheat furnace, such as, forexample, billets, blooms, slugs or slabs, or any other product used byiron and steel manufacturers in their production line (such as sheet orplate, tube, etc.). The invention relates more particularly to a methodof treating a metallurgical product in a furnace, in which the productbe treated is introduced into the furnace and then subjected to thedesired treatment before being removed from the furnace, the furnacecomprising heating means and especially burners for raising the variouszones of the furnace to a variable temperature, it being possible forthe atmosphere in these various zones to have an identical or differentcomposition depending on the zones in question of said furnace.

2. Related Art

The environment of a steel (or any other product, especially a metal oriron or steel product), when it is raised to a high temperature during aheat treatment, is often an atmosphere which is oxidizing with respectto the metal. This situation may result, on the one hand, in oxidationof the metal with the formation of a surface layer of scale and, on theother hand, in decarburization of the steel with the creation of acarbon concentration gradient near the surface of the workpiece.

The altered region at the surface of these workpieces is essentiallycomposed of two parts (see FIG. 1), one lying on the atmosphere side(upper scale) and the other adjacent the metal (hybrid region).

The upper part generally is composed of three dense oxide layers: alayer of oxide Fe₂O₃ (hematite), which is very thin (with a thickness ofa few microns), a layer of magnetite (Fe₃O₄) (about 4% of the totalscale) and a thick layer of the oxide FeO (wustite) (about 95% of thetotal scale) which is of greater or lesser porosity depending on thereheat time and the reheat temperature.

The growth of this scale, which follows a parabolic law, is controlledby the diffusion of Fe²⁺ ions into the wustite and the magnetite and bythe diffusion of oxygen O₂ ⁻ into the hematite.

The lower part, or hybrid region, has a greater or lesser thicknessdepending on the nature of the steel. It is located at the metal/scaleinterface and consists of a mixture of FeO and products resulting fromthe reaction of FeO with the oxides of certain alloying elements. Thislower part is also composed of a metal region altered by variousphenomena, such as decarburization or internal oxidation.Decarburization is a phenomenon involving the solid-state diffusion ofcarbon, which reacts with the FeO scale (and/or H₂O). The permeabilityof industrial scale to the gaseous products resulting from the oxidationof carbon (especially CO) makes this oxidation at the surface of themetal almost immediate. Decarburization is therefore limited by thediffusion of carbon at the treatment temperature and is favored by theability of the gases formed (CO) to escape from the scale-steelinterface.

Depending on the thermal profile imposed and on the composition of theatmosphere (especially the O₂, H₂O and CO₂ contents), the iron or steelproducts may be oxidized (scale) and decarburized (this being the moreso in the case of high-carbon steels). In both cases, the steelmanufacturer will have to subject his workpieces to an additionaloperation aimed at eliminating these surface defects. Whereas the oxidelayer may be removed by various descaling techniques, the decarburizedlayer, that forms an integral part of the workpiece, cannot be easily“erased”: the surface of the product is devoid of some of its carbonatoms, thereby degrading the mechanical properties on the surface of theproduct (longevity, hardness, etc.).

The oxidation or decarburization of steel in a reheat furnace thusresults in a loss of raw material, which is called “loss on ignition”,and a degradation of the surface properties of products, which areprejudicial to the steelmaker.

A major constraint, which will also affect the final quality of theproduct at the end of the reheat process, is the final temperature ofthe product and its thermal homogeneity, this being so whatever theheating history that has taken place in the furnace (time spent atcertain temperature levels, slower production rate following a rollingmill incident, etc.). Any lack of thermal inhomogeneity will causestructural defects and a posteriori mechanical embrittlement of thefinished products. These defects may also cause certain parts of therolling mill (especially rolling-mill stands) to be stopped or evenbroken.

Any optimization of the metallurgical quality of the product must meetthis constraint with regard to the thermal homogeneity of the product.During operation of the furnace by the operator, control of andcompliance with the temperature rise of the product are key factors inensuring in the end that the thermal homogeneity constraint is met.

A person skilled in the art knows that, to avoid decarburization andoxidation, it is recommended to work in a protected atmosphere bysubstoichiometric combustion (using a fuel-rich mixture generating aneutral or even reducing atmosphere with respect to steel). This methodis employed in galvanizing processes (see, for example, Galvanisation etaluminiage en continu [Continuous galvanizing and aluminizing] by E.Buscarlet, Technique de l'ingénieur [Engineering Techniques], 1996.

It is also known, from U.S. Pat. No. 4,415,415, to treat products in anatmosphere containing at least 3% oxygen by volume, and to do so overthe entire length of the furnace, thereby inevitably resulting in theformation of scale but making it possible to control the quality of thescale, which, under these conditions, becomes non-adherent and easilyremovable.

Patent EP-A-0 767 353 also proposes to vary the furnace atmosphere byzoning the furnace, that is to say by isolating the furnace into severalchambers within which a highly oxidizing atmosphere is recommended, soas to be able to control the formation and quality of the scale. In thiscase, the loss on ignition is not reduced, but on the contrary isincreased, only the quality of the scale being controlled.

The various methods known from the prior art therefore suggest that theproducts either be treated in an oxidizing atmosphere or in a reducingatmosphere.

The use of these various methods also has an additional drawback in thecase of the treatment of steel products. This is because it is importantto be able to measure the oxidizing or reducing character of theatmospheres involved. The only information available duringimplementation of these processes is provided by measurement probeslocated either in the roof, that is to say far from the surface of theproducts, or in the flue of the furnace. These measurements aretherefore not representative of the composition of the atmosphere whichinteracts directly with the product. In general, the only measurableparameter of the atmosphere is the oxygen content. This information isgenerally insufficient—because the fact that the amount of oxygen in thesmoke leaving the furnace is zero does not necessarily mean that thefurnace atmosphere in contact with the metal workpieces is reducing withrespect to steel (see, for example, Combustion Engineering and GasUtilisation, published by British Gas, 1992, page 23). According to theApplicant, the species H₂O and CO₂ also have an oxidizing role on thecharge and are involved in scale formation reactions and indecarburization mechanisms. At the present time, it is not known how tomeasure these species simply and quickly.

To operate the furnace and meet the final constraint of thermalhomogeneity of the product, the operator adopts an initial temperatureprofile of a given product for a given furnace, depending on the type ofcharge and of production. This profile is either known to the operator,because of his know-how, or is calculated from charts, or elsecalculated using suitable software.

The only information available for the operator and/or the furnaceoperation software are the measurements delivered by one or morethermocouples located in the roof of the furnace. These thermocouplesare placed far from the charge and are not representative of the heatflux received by the charge beneath the burners. It is thereforenecessary to estimate the relationship which links the roof temperature(which is measured) and the temperature of the charge (usefulinformation). This relationship is either empirical (based on theoperator's know-how) or calculated using furnace operation software.

Not only is this measurement only an indirect measurement of thenecessary information, but the estimated relationship may prove to beless and less accurate upon aging of the furnace, of the thermalcharacteristics of the various charges and variations in the type offuel used.

Finally, this measurement is a measurement at a certain point, usuallylocated on the axis of the furnace and it does not take into accountpossible variations in said parameter over the entire width of thefurnace.

The fact of not having measurements made as close as possible to theproduct has the consequence that the characteristic times of the processfor heating these products is not known exactly. Yet it has been foundthat these characteristics have a major influence on the oxidation anddecarburization kinetics of said products, it being possible that anincorrect estimation of these times has serious consequences as regardsthe final metallurgical quality of the product.

SUMMARY OF THE INVENTION

The aim of the present invention is to provide a method of operating afurnace (temperature, composition of the atmosphere) and an associatedcontrol procedure, making it possible to optimize both the metallurgicalquality of a product and the loss on ignition and thermal efficiency ofa furnace.

FIGURES

For a further understanding of the nature and objects for the presentinvention, reference should be made to the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich like elements are given the same or analogous reference numbersand wherein:

FIG. 1 shows an altered region at the surface of the workpiece which isessentially composed of two parts.

FIG. 2 shows a characteristic curve of the variation in temperature ofthe product as a function of time, controlled according to the method ofthe invention.

FIG. 3 shows the application of the invention to a reheat furnace.

FIG. 4 shows the control, according to the invention, of the temperaturerise of the product.

FIG. 5 shows a curve of the temperature in a reheat furnace as afunction of time.

FIG. 6 shows a curve of the variation in the amount of scale as afunction of time.

FIG. 7 shows another curve of the variation in the amount of scale as afunction of time.

DESCRIPTION OF PREFERRED EMBODIMENTS

The method according to the invention makes it possible to avoid theaforementioned drawbacks and allows the abovementioned aim to beachieved.

The method according to the invention is characterized in that theproduct to be treated has a temperature that increases between themoment when it is introduced into the furnace and the moment when it isremoved therefrom, the temperature rise curve having a slope thatincreases over a first time interval between the time t₀ of introductionof the product into the furnace and the time t₁ at which the productachieves a surface temperature of 650° C., an approximately constantslope between the time t₁ and the time t₂ at which the product reaches atemperature about 15% below the desired final surface temperature of theproduct to be treated when it leaves the furnace, then a slope thatdecreases between the time t₂ and the time t₃ at which the product to betreated leaves the furnace, in which method the heating power of thefurnace is increased relative to its power when only air/fuel burnersare used, so as to increase the slope of the curve giving the rise intemperature of the product to be treated, at least during certainperiods of treatment of the product in the furnace between the times t₁and t₂, thereby reducing the duration of the treatment of the product tobe treated and correspondingly reducing the thickness of thedecarburized layer and/or the layer of scale formed on the surface ofthe product.

Preferably, the increase in the heating power of the furnace is obtainedby means of oxyfuel burners that constitute at least part of the heatingmeans of the furnace, especially part of the heating means of thefurnace corresponding to the zone reached by the product between thetimes t₁ and t₂. It is also possible place this or these oxyfuel burnersin a zone adjacent the abovementioned zone, which would make it possiblefor the same increase in power (in said zone reached by the productbetween the times t₁ and t₂) to be obtained indirectly.

In general, the oxidizer delivered to the oxyfuel burners, constitutingat least part of the heating means of the furnace, contains at least 88%oxygen, preferably greater than 90% oxygen and even more preferablygreater than 95% oxygen.

In general it is found that the time for treating the product betweenthe temperatures of 700° C. and 800° C. reached by the surface of theproduct is reduced by 15% to 50% of its reference value, preferably by20 to 35% of its value, whereas the treatment time between thetemperatures of 700° C. and the final temperature of the surface of theproduct is reduced by between 3 and 25% of its reference value,preferably between 7 and 15% of its reference value.

Preferably according to the invention, used by itself or in combinationwith the other variants of the invention, the atmosphere of the furnacevaries along the length of the furnace as a function of the skintemperature of the metallic product.

According to a first variant of the invention, used alone or incombination with the other variants of the invention, the atmosphere ofthe furnace on contact with the product to be treated contains about 0.5to 5 vol % oxygen and preferably between 1.5 to 4 vol % oxygen when theskin temperature T at the surface of the treated product is greater thanor equal to the equalization temperature T_(equalization), which isequal to 85% of the temperature at the surface of the product (dischargetemperature) as it leaves the furnace. Preferably, the equalizationtemperature T_(equalization) is equal to 90% of the dischargetemperature.

According to another variant of the invention, used by itself or incombination with the previous ones, the atmosphere on contact with theproduct to be treated has an oxygen concentration of less than a fewhundred ppm and a CO concentration of between 0.1 and 15 vol %,preferably 0.5 to 5 vol %, when the skin temperature T at the surface ofthe product is above 700° C. and below the equalization temperature ofthe product, defined as being equal to 90% of the skin temperature ofthe product as it leaves the furnace.

According to yet another variant of the invention, used by itself or incombination with the previous ones, the atmosphere in contact with theproduct to be treated has an oxygen concentration of between 0.5 and 4vol % and preferably between 2 and 3 vol % when the skin temperature Tat the surface of the product to be treated is below 700° C.

The invention allows the metallurgical quality of products to beoptimized by optimizing the heating profile in the furnace together withimproved control of the composition profile of the atmosphere in thefurnace. This control continuously monitors the O₂ and/or H₂O and/or CO₂contents of the atmosphere in the various zones of the furnace, and/orthe temperature at the surface of the products to be treated, willpreferably be carried out using a diode laser. This TDL (Tunable DiodeLaser) system makes it possible in fact to measure the averageconcentrations of gaseous species along the length of the optical pathof the laser beam. For further details about diode lasers and inparticular TDL-type diode lasers, reference may be made to the articleby Mark G. Allen entitled “Diode Laser Absorption Sensors for GasDynamic and Combustion Flows”, Mes. Sci. Technology, 9, 1998, pages 545to 562, and incorporated in the present text as reference. In general,these diode lasers are laser radiation sources, some of which operate atroom temperature while others must be cooled. The laser beam emitted canin general be tuned within a wavelength range by varying the currentinjected into the laser source. All that is then required is to chooselaser beam sources that can be tuned within wavelength ranges whichcorrespond to at least one of the characteristic lines of the absorptionspectrum of the species which it is wished to detect. Preferably, thediode laser will be placed near the surface of the products, at adistance varying between 1 mm and 15 cm, preferably between 2 cm and 6cm. It is in the region of the surface of the product that the O₂, H₂Oand CO₂ partial pressures thus of the temperature are involved in themechanisms described above, namely scale formation and decarburization.This monitoring as close as possible to the surface also makes itpossible for predictive tools to be developed and for the methodproposed to be implemented properly.

A greater understanding of the invention will be gained from thefollowing illustrative examples, given without implying any limitation,in conjunction with the figures which show:

FIG. 2 shows a characteristic curve of the variation in temperature ofthe product as a function of time, controlled according to the method ofthe invention;

FIG. 3 shows the application of the invention to a reheat furnace;

FIG. 4 shows the control, according to the invention, of the temperaturerise of the product;

FIG. 5 shows a curve of the temperature in a reheat furnace as afunction of time;

FIG. 6 shows a curve of the variation in the amount of scale as afunction of time;

FIG. 7 shows another curve of the variation in the amount of scale as afunction of time.

In FIG. 2, the curve (21) represents the heat-up curve of the product,for example the skin temperature of a billet or of a slab in a reheatfurnace. According to this curve, it is possible to define the times t₀,t₁, t₂ and t₃ corresponding, respectively, to the time t₀ when theproduct is introduced into the furnace, to the time t₁ when the skintemperature reaches 650° C., to the time t₂ when the skin temperature isequal to 85% of the final (or discharge) temperature T_(out) of the skinof the product and, finally, to the time t₃ when the product isdischarged at its final temperature T_(out). Thus, a time interval Δ₁corresponding to the time that the surface of the product spends betweent₁ and t₂ is defined. A time Δ₂ corresponding to the time spent by theproduct between t₁ and t₃ may also be defined.

The method according to the invention consists in reducing the time Δ₁by about 8% to 40% of its reference value and preferably by about 10% to30% of its reference value. This allows the thickness of thedecarburized layer to be decreased by at least 20%, depending on thecontents of the alloying elements and specifically the carbon content,compared with the method of the prior art using either the empiricaloperation of the furnace by an experienced person skilled in the art orthe operation of the furnace using temperature charts or suitablesoftware. It is in particular the reduction in the time Δ₁, resulting inan increase in the slope of the curve 52 compared with the slope of thecurve 51 between the times t₁ and t₂ corresponding to the temperaturesof 650° C. and of 85% of the skin temperature at the exit of thefurnace, which is fundamental according to the method of the invention,as it has been demonstrated that it is in these temperature ranges thatit is necessary to increase the slope of the heat-up curve of theproduct if it is desired to obtain the hoped-for reductions.

Likewise, the invention makes it possible to reduce the time Δ₂ bybetween 5 and 30% of its reference value and preferably by between 7 and15% of its reference value. This makes it possible to decrease the massof the scale by between 5 and 30%, depending on the nature of the steel.

This reduction in the times Δ₁ and Δ₂ is achieved, according to theinvention, by increasing the energy transferred to the productthroughout the duration of its residence in the furnace. This may beachieved by increasing the available energy (by adding an energy source,via naked-flame burners, radiant tubes or else electrical resistanceelements or induction heating) or by increasing the efficiency of theavailable energy (by enriching the combustion air up to, for example,oxygen, up to a purity of up to 100%), preferably to above 90 vol % OfO₂.

The maximum reduction of Δ₂ is fixed by having to meet the constraint ofthermal homogeneity of the product on leaving the furnace, thisconstraint itself being governed by the thermal conduction within theproduct.

Compared with a given reference situation (given furnace and givenhourly production, and therefore given run speed, of given products),the reduction in times Δ₁ and Δ₂ corresponds either to a shortening ofthe furnace or to an increase in the run speed of the products.

A second aspect of the invention consists in controlling the compositionprofile of the species of the atmosphere in the furnace and along theentire length of the path traveled by the product through the furnace.

As a matter of fact, the composition of the atmosphere, that is to sayespecially the contents of the oxidizing components (O₂, H₂O, CO₂) inthe atmosphere, is a parameter which has an impact on the metallurgicalquality of the product. Thus, for a given thermal profile, it ispossible to optimize the quality of the product by maintaining a higheror lower oxygen content depending on the furnace zone in question.

In FIG. 3, which shows a reheat furnace, the direction in which theproducts (35) run and the flow direction of the smoke are indicated.Curve (30) is the curve showing the temperature rise of the product.

As the charge (35) runs through the reheat furnace, it undergoes a firsttemperature rise in the zone (32). The temperatures then reach atemperature T_(decarb). This temperature is typically 700° C. in thecase of steels and the sensitivity of the decarburization to thistemperature is greater the higher the carbon content of the steel. AboveT_(decarb), and in the presence of oxidizing species, thedecarburization and scale formation reaction rates increase: thetemperature at which scale formation becomes effective is about 800° C.in the case of steels. The product passes through the zone (33) and thenenters the equalizing zone (34), when the product is at the temperatureT_(equalization) (typically 1100° C.). This zone, at very hightemperature, brings the product to its final temperature (T_(final),typically 1200° C.), and is particularly critical for the formation ofscale.

Three ports for installing a diode laser are provided on this furnace.The port (36) is located in the equalizing zone (34), the port (37) islocated in the heating zone (33), the port (38) is located in the zone(32) which contains the zone called the recovery zone, whereas the port(39) is located in the flue (31).

According to the invention, the concentration of the oxidizing speciesis measured by the ports (36), (37), (38), (39), each port receiving alaser beam (via an optical fiber), or a laser beam emitter, a receiverbeing provided in the opposite wall of the furnace (or else a mirrorwhich sends the beam back parallel to the incident beam, the receiverbeing placed beside the emitter).

In the zone (32) (temperature below T_(decarb)), the fuel and oxidizerflow rates for the burners in the zone (32) must be adjusted, accordingto the invention, so as to create an oxygen content in the atmosphere inthis zone (32), measured by the corresponding diode laser, of between0.5 and 4 vol % and preferably between 2 and 3 vol %.

If the equalizing zone (32) is not fitted with burners, this correctionmay be made by the addition of oxidizer via lances, for example oxygenlances, the amount injected being controlled by the measurement of theoxygen content by the diode laser.

The measurement is preferably carried out as close as possible to theproduct, either via the port (38) in this zone (32) or via the port(39), that is to say in the smoke extraction duct where the same oxygencontent is monitored. If the measurement shows a lack of oxygen, thislack must be corrected by regulating the burners, hence increasing therate of flow of oxidizer (oxygen) to the burners of the zone (32) or ofthe preceding zone.

In zone (32), a protective layer of Fe₂O₃ and Fe₃O₄ will be formed andreinforced by the presence of residual oxygen in the smoke. These oxideswill be formed to the detriment of more plastic oxides such as FeO orFeSiO₄ which in this case result in strong adhesion of the scale. Inaddition, at low temperature, the protective conditions (in theparabolic stage of the oxidation) are established more quickly foroxygen partial pressures lying within the aforementioned range (0.5 to 4vol %).

In the zone (33) (temperature above T_(decarb) but belowT_(equalization)), the fuel and oxidizer flow rates for the burners inthe zone (33) must be regulated according to the invention so as toproduce an oxygen content close to zero in the atmosphere. Theatmosphere will be depleted in oxygen, and therefore the fuel, and inparticular the CO, will be in excess. Thanks to the measurement carriedout via the port (37), the burners will be regulated in such a way thatthe O₂ concentration is close to zero and the CO concentration isbetween 0.1 and 15 vol % and preferably between 1 and 10 vol %. In thishigher-temperature zone, it is desired to limit scale formation anddecarburization as much as possible, by reducing the concentration ofthe oxidizing species (O₂, CO₂, H₂O).

In the zone (34) (temperature above T_(equalization)) the fuel andoxidizer flow rates for the burners in the zone (34) must be regulatedaccording to the invention so as to produce an oxygen content in theatmosphere of between 0.5 and 5 vol % and preferably between 1.5 and 4vol %. The measurement of this concentration is made as close aspossible to the product, between 1 mm and 15 cm therefrom, via the port(36). In this zone and in the presence of oxygen, there is consumptionof the decarburized layer by oxidation, which will be accompanied by anincrease in porosity of the scale, which will facilitate its removaloutside the furnace.

The port (39) is used to check at all times the CO concentration and theO₂ concentration in the smoke before it is discharged.

When the atmosphere is controlled in this way, according to theinvention, the mass of scale is reduced by between 5 and 25%, dependingon the nature of the steel.

Likewise, as a general rule it may be noted that the thickness of thedecarburized layer is reduced by at least 10%, depending on the contentsof the alloying elements and specifically the carbon content.

The gains obtained by controlling the atmosphere are concurrent with thegains made by reducing the times Δ₁ and Δ₂ described above.

FIG. 4 illustrates the monitoring according to the invention of thetemperature rise of the product. The invention consists in monitoringthe temperature rise of the product and in regulating the burners, bymeans of a local measurement, zone by zone and a few cm above thecharge, of the temperature of the atmosphere in the furnace using adiode laser system.

FIG. 4 shows, in the furnace (41), the position of the product (42) andof the thermocouple (48) according to the prior art. The measurement bythe thermocouple (48) gives a temperature value on the axis of thefurnace but far from the product (42).

According to the invention, one or more diode lasers are fitted in orderto measure an average temperature value along the optical path over thewidth of the furnace. Such an arrangement allows:

-   -   an average measurement to be made along the furnace, this being        more representative of the product than a discrete measurement        in the roof;    -   a measurement close to the product, and therefore directly        associated with the surface temperature of the product which is        in equilibrium with the temperature of the gas in contact with        the said surface;    -   quantification of the relationship between roof temperature and        product temperature, which in the prior art was established        empirically (by retaining the roof thermocouple).

In FIG. 4, the number of measurement points has been limited here tothree. Preferably, between 1 and 10 measurement points in a furnace willbe used.

The furnace (41) is fitted with ports (43, 44, 45) located above theproduct (42).

The furnace operator must comply as closely as possible with thetemperature rise profile (47) of the product. This profile is suppliedto the operator, either through his experience, or by means of a chartor via furnace operation software.

To control the product temperature rise (47), a person skilled in theart hitherto had available only the curve (46) indicating the rooftemperature along the axis of the furnace, the thermocouple (48) ofwhich delivers, for example, a measurement point as illustrated on thecurve. According to the invention, a person skilled in the art now canobtain measurements located along the curve (47) which are directlyassociated with the surface temperature of the product. The operator cantherefore vary the power of the burners in order to find the desiredtemperature level on the curve (47). If the measured temperature is toolow, the operator will then increase the heating power in the zone closeto the measurement point. Conversely, if the measured temperature is toohigh, the operator will then reduce the power in the zone close to themeasurement point.

The invention also has the following advantage: Certain furnaces usesoftware called “Niveau 2 [Level 2]” to reproduce, whatever the heatingconditions, a product temperature rise, according to a given initialprofile. Until now, a person skilled in the art did not have availableany measurement for continuously confirming the effect of the software.It is another aspect of the invention that this software is coupled withthe direct measurements of the product according to the invention,thereby making it possible to systematically verify in real time theintended temperature of the product.

EXAMPLES Example 1

A first illustrative example is described with the aid of FIG. 5, whichshows the heating curve (51) associated with a long billet reheatfurnace. The combustion is carried out using burners, the fuel for whichis natural gas and the oxidizer for which is preheated air, beforeimplementation of the invention. (In this FIG. 5, the parameters t₁, . .. and Δ₁, . . . are in parentheses when they relate to curve 51according to the prior art and are without parentheses when they referto curve 52).

Implementation of the invention is characterized by replacing theexisting burners, the oxidizer for which is air, with burners for whichthe oxidizer has an oxygen concentration of greater than 21 vol %, andpreferably greater than 88 vol %. More preferably, the oxidizer will beindustrially pure oxygen. The associated heating curve is the curve(52). It should be noted that the times Δ₁ and Δ₂ are reduced from 2100to 1700 seconds and from 5300 to 4800 seconds, respectively. Themetallurgical quality of the method obtained according to the curve (52)will be greatly enhanced by monitoring the heating curve in FIG. 5, withthe installation of diode lasers at the locations explained with regardto FIG. 3 and FIG. 4 or any measurement means allowing this heatingprofile to be suitably controlled.

FIG. 6 shows the amount of scale produced using the method describedabove. The amount of scale (61) is associated with the referencesituation and the scale curve (62) is associated with the implementationof the invention. The two curves have been normalized with respect tothe maximum value of the scale thickness obtained under the conditions(61).

Implementation of the method according to the invention, which reducesΔ₁ by 19% and Δ₂ by 9.5%, makes it possible to reduce the amount ofscale by 8% on average (FIG. 6). Depending on the experiments, thethickness of the decarburized layer is reduced by between 9 and 17%.

Example 2

The illustrative example below was implemented in a billet reheatfurnace having a power of 33 MW and a length of about 30 m. The burnersoriginally present in the furnace were burners called air-fuel burners,the combustion air being preheated to 300° C.

FIG. 7 compares, for an identical heating profile, the amount of scaleproduced (curve 71) with a heating atmosphere whose oxygen concentrationin the wet smoke is constant and equal to 3.5 vol %, and the amount ofscale produced (curve 72) with a heating atmosphere whose oxygenconcentration in the wet smoke varies in the following manner:

-   -   about 1.5% O₂ (to within 20%) when the skin temperature T is        above the equalization temperature T_(equalization) (defined as        being between 85% and 90% of the discharge temperature);    -   about 0% O₂ (up to a few hundred ppm) and a CO concentration of        between about 0.5 and 3% (to within 20%) for        T_(decarb)<T<T_(equalization), T_(decarb) being the        decarburization start temperature (700° C.); and    -   about 2% O₂ (to within 20%) when the skin temperature T is below        T_(decarb).

The mean O₂ concentration in the smoke may be measured by a standardoxygen probe, but it may be preferable to employ a diode laser (of the“TDL” type), the beam of which passes at a distance of less than about 6cm from the treated product, for fine monitoring, in real time, of avariation in concentration of the species above at the surface of theproduct so as to better meet the atmosphere profile set in order tomatch the heating profile.

According to this Example 2, implementation according to the inventionallows the thickness of the scale to be reduced by 11% (FIG. 7).Depending on the experiments, the thickness of the decarburized layer isreduced by between 12 and 20%.

It will be understood that many additional changes in the details,materials, steps and arrangement of parts, which have been hereindescribed in order to explain the nature of the invention, may be madeby those skilled in the art within the principle and scope of theinvention as expressed in the appended claims. Thus, the presentinvention is not intended to be limited to the specific embodiments inthe examples given above and/or in the figures.

1. A method for treating a metallurgical product in a furnace,comprising the steps of: i) introducing said product into said furnaceat time t₀; ii) subjecting said product to the desired treatment beforebeing removed from said furnace at time t₃; iii) increasing thetemperature of said products to about 650° C. during the period (t₁-t₀),wherein t₁ represents the time at which the surface temperature isreached; iv) increasing the temperature of said product almost uniformlyto about 85% of the desired final temperature (T_(equalization)) duringthe period t₂-t₁, wherein t₂ represents the time at which thetemperature of said product is reached; v) increasing the temperature ofsaid product at a decreasing rate to said desired final temperatureduring the period (t₃-t₂); and vi) enhancing the metallurgical qualityby reducing the thickness of scales or decarburized layer formed on thesurface of said product.
 2. The method according to claim 1, whereinsaid method further comprises: vii) isolating said furnace into variouszones of identical or different composition; viii) raising thetemperature of the zones to a variable temperature by heating viaburners; and ix) increasing the heating power relative to only whenair/fuel burners are utilized.
 3. The method according to claim 2,wherein said heating power is generated by oxyfuel burners thatconstitute at least part of the heating means of the furnace.
 4. Themethod according to claim 3, wherein said heating power corresponds tothe zone that ranges from time t₁ to t₂.
 5. The method according toclaim 3, wherein said method further comprises the step of: deliveringan oxidizer to the oxyfuel burners, and wherein said oxidizer comprisesat least about 88% oxygen.
 6. The method according to claim 5, whereinsaid oxidizer comprises greater than about 90% oxygen.
 7. The methodaccording to claim 6, wherein said oxidizer comprises greater than about95% oxygen.
 8. The method according to claim 1, wherein the methodfurther comprises: reducing the time for treating the product from about700° C. to about 800° C. by about 15% to about 50% of the referencevalue, and wherein said reference value corresponds to the temperaturevalue of the prior art.
 9. The method accord according to claim 8,wherein said time is reduced from about 20% to about 35% of thereference value.
 10. The method according to claim 1, wherein the methodfurther comprises the step of: reducing the time for treating theproduct from about 700° C. to the final temperature by about 3% to about25% of the reference value, and wherein said reference value correspondsto the temperature value of the prior art.
 11. The method accordaccording to claim 10, wherein said time is reduced from about 7% toabout 15% of the reference value.
 12. The method according to claim 2,wherein the temperature of the furnace's atmosphere is based on thesurface temperature of the metallurgical product.
 13. The methodaccording to claim 12, wherein said atmosphere comprises from about 0.5%to about 5 vol % oxygen.
 14. The method according to claim 12, whereinsaid furnace comprises: i) about 1.5% to about 4 vol % oxygen in saidatmosphere; and ii) surface temperature T that is greater than or equalto said T_(equalization), and wherein said T_(equalization) is equal toabout 85% of the surface temperature T of the product as it leaves thefurnace.
 15. The method according to claim 14, wherein saidT_(equalization) is equal to about 90% of the discharge temperature, andwherein said discharge temperature is the temperature of the product asit leaves the furnace.
 16. The method according to claim 1, wherein theatmosphere comprises: i) an oxygen concentration of less than a fewhundred ppm; ii) a CO concentration from about 0.1% to about 15% volwhen the surface temperature is above about 700° C. and below saidT_(equalization) of the product, and wherein said T_(equalization) isequal to about 90% of the discharge temperature.
 17. The methodaccording to claim 16, wherein said CO concentration ranges from about0.5% to about 5% vol.
 18. The method according to claim 1, wherein theatmosphere comprises an oxygen concentration that ranges from about 0.5%to about 4% vol when the surface temperature is below about 700° C. 19.The method according to claim 18, wherein said oxygen concentrationranges from about 2% to about 3% vol.
 20. The method according to claim1, wherein the method further comprises means of analyzing at least oneparameter of the atmosphere by utilizing a diode laser, and wherein thebeam of said laser is located at a minimum distance ranging from about 1cm to about 6 cm to the product's surface.
 21. The method according toclaim 1, wherein said method comprising the steps of: i) introducingsaid product into said furnace at time t₀; ii) subjecting said productto the desired treatment before being removed from said furnace at timet₃; iii) increasing the temperature of said products to about 650° C.during the period (t₁-t₀), wherein t₁ represents the time at which thesurface temperature is reached; iv) increasing the temperature of saidproduct almost uniformly to about 85% of the desired final temperature(T_(equalization)) during the period t₂-t₁, wherein t₂ represents thetime at which the temperature of said product is reached; v) increasingthe temperature of said product at a decreasing rate to said desiredfinal temperature during the period (t₃-t₂); and vi) enhancing themetallurgical quality by reducing the thickness of scales anddecarburized layer formed on the surface of said product.